Travelling wave tubes



` L.; M, FIELD TRAVELLING WAVE TUBES 2 Sheets-Sheet l Filed June 9, 1950INVENTQR ESTER M. F/ELD BY af/ ATTORNEY 1 L. M. FIELD TRAVELLING WAVETUBES Aug 21. 1956 2 Sheet's s`heet 2 Filed June 9' 1950 A INVENTORESM-RM HELD A'II'TORNEY United States Patent O TRAVELLING WAVE TUBESLester M. Field, Palo Alto, Calif., assignor to The Board of Trustees ofThe Leland Stanford Junior University, Stanford University, Calif., alegal entity having corporate powers of California Application June 9,1950, Serial No. 167,150 19 Claims. (Cl. S15-3.5)

This invention relates to electron discharge devices, and moreparticularly to improvements in travelling wave tubes, whereinamplification of electromagnetic wave energy is effected by interactionbetween a travelling wave and a stream of electrons moving with theWave.

In the operation of such tubes, it is generally necessary that theelectrons and the wave to be amplified travel at approximately the samevelocity. This is accomplished by the use of some kind of slow wavepropagating structure, such as a helix. A typical prior art deviceincludes a relatively long small diameter helical winding which issupplied at one end with the input energy in such manner as to carry aslowly travelling wave. An electron stream is directed along thelongitudinal axis of the helix, and amplilied energy appears at theother end of the helix.

To obtain a substantial amount of amplification with such tubes, it isnecessary that the helix or equivalent slow wave propagating structurebe made relatively long, in order that the wave and the electrons mayinteract throughout a substantial distance. This requirement leadsdirectly to the problem of focussing the electron stream into a thinbeam, and keeping the beam together against the forces of mutualrepulsion of the electrons.

The power handling capability of the tube is limited by the beamcurrent, which is proportional to the product of the beam density andthe cross sectional area of the beam. The maximum attainable density isnot high, on account of the dificulty in focussing, and the maximumusable beam cross section depends upon the size of the helix, which islargely determined by the operating frequency range. Thus, in prior arttubes wherein the Wave and the electrons move along the same axis, boththe gain and the power output are limited, and as a practical designmatter a compromise must be made between them. Relatively highaccelerating voltages must be used, and more or less elaborate focussingmeans, such as external magnets, are required.

Another usually undesirable characteristic of the above described priorart tubes is that while they exhibit gain in the forward direction, i.e. in the direction of electron motion, they also propagate waves in theother direction. The amplified waves appearing at the output end of thedevice may feed back this way to the input end, resulting in sustainedand uncontrolled oscillation. This can be, and has been, avoided byintroducing losses in the helix, but such losses also reduce the desiredgain in the forward direction.

It is one of the principal objects of the present inven-v tion toprovide improved types of travelling wave tubes wherein the foregoingdisadvantages are avoided.

More specifically, it is an object of this invention to providetravelling wave tubes in which the direction of electron flow istransverse to the general direction of wave propagation.

Another object of the invention is to provide tubes of the describedtype which may be designed to make the Wave-electron interaction spaceof any desired length,

without the introduction of any stringent beam forming or focussingrequirements.

A further object of the invention is to provide tubes of the describedtype which are simple and rugged in construction and do not involveclose mechanical tolerances.

Other objects and advantages of the invention will become apparent fromthe following description with reference to the accompanying drawings,wherein:

Fig. l is a longitudinal section of a presently preferred embodiment,

Figs. 2 and 3 are cross sections of the structure of Fig. 1 in theplanes 2--2 and 3-3 respectively,

Fig. 4 is a plan View showing the internal structure of a modificationof the device of Fig. l,

Figs. 5 and 6 are side and end elevations respectively of the structureof Fig. 4,

Fig. 7 is a transverse section of a modification of the device of Fig.4,

Fig. 8 is a plan view of another modification of the device of Fig. 1,and

Fig. 9 is a transverse cross section of the structure of Fig. 8

The travelling wave tube shown in Figs. l, 2 and 3 includes a slow wavepropagating structure in the form of a skewed conductive helix 1, ofoblong or flattened cross section. The helix 1 may be made of copperwire Wound on an insulating form 5 provided with notches 7 for holdingthe wire in place.y The pitch of the helix is relatively small, i. e. itis the space between adjacent notches 7, but through the major portionof the helix the wire is laid at a relatively large angle to the normalpitch line so that the lower side of each turn is considerably advancedwith respect to the upper side. The last few turns near each end of thestructure are laid at progressively decreasing angles to provide asmooth transition from the skewed winding to a normal helix.

The form 5 is supported at its ends by transverse `plates 9, which arein turn supported between slightly bowed sheet metal members 11. Themembers 11 extend across tubular conductive sleeves 13, each disposedwithin and near a respective end of a tubular vacuum tight glassenvelope 15.

A linear electron gun assembly 17 is provided near the lower edge of thewave propagating structure 1 and slightly in front thereof, as shown inFig. 3. The assembly 17 includes a strip cathode member 19, focussingelectrodes 21, and an accelerating electrode 23. The cathode 19 isadapted to be heated by a heater element 25. The assembly 17 issupported by conductive posts 27, 28, 29 and 30 which extend through andare sealed to the wall of the envelope 15, and act as terminals for theapplication of heater and accelerating potentials.

A collector electrode 31 parallel to the cathode assembly 17 issupported near the upper edge of the structure 1 by a post 33 whichextends through and is sealed to the wall 15. Similar terminals 35 arebrought out through the wall 15 from the sleeves 13. The ends of thehelix are connected to conductors 37 and 39 respectively, which extendthrough seals in the Wall 15 and into external conductive sleeves toform coaxial line terminals 41 and 43. The external sleeves aresupported by sleeves 45 which closely surround the envelope 15 injuxtaposition with the sleeves 13.

The operation is substantially as follows: The heater 25 is energizedfrom an external source, not shown to` cause emission of electronls fromthe cathode 21. Another external source, such as a battery, is connectedto the accelerating electrode 23 to maintain said electrode at apositive potential with respect to the cathode 21. The collectorelectrode 31 may be connected to the same point as the acceleratingelectrode 23 or may be supplied ,with another potential, also positivewith respect to the Patented Aug. 21, 1955` cathode. The assembly 17produces a relatively thin sheetalike stream of electro-ns flowing tothe collector 31 substantially at right angles to the longitudinal axisof the propagating structure 1. Y

High frequency energy to be ampliiied is applied.y as by means of acoaxial line to the terminal 41, and it travels along the wire of thehelix at approximately the velocity of light in free space. Since theinput wave energy must travel. throughout. the lengthv 'of a full` turnof the helix in order to advance, the distance between two adjacentturns in the longitudinal direction of the propagating Lstructure 1, theefect is that of causing the wave to propagate relatively slowly alongthe helix. The phase of the Wave is substantially the same throughoutthe length of'any given lineal turnelement of thel hel-ix because thewave propagates along the wire at the. velocity of light. Thus in theuniformly skewed portion. of the helix, an electron travelling from thecathode assembly I7 to the collector 31 will encounter the waveV atprogressively retarded space phases, as it crosses4 successive turns ofthe wire. Another way of stating the same thing is that the phase frontofthe wave is skewed o'r `tilted so that the wave has a component ofpropagation velocity perpendicular to the longitudinal axiszof thepropagating structure 1 and parallel to the direction of electron. flow.

The accelerating potential is adjusted to make the electrons travel'across the structure 1` at substantially the same velocity as; thetransverse component of wave velocity. Thus the electrons interact withthe wave substantially as in the conventional helix, type travellingwave tube, delivering energy to the Wave as they travel acrosssuccessive Vturns of 4the helix. 'The ampliiiedv wave at the upper endof each helix turn, for example at the point 47, appears at the lowerend 49 of the same turn adjacent a different element of the electronstream. Accordingly' the wave as amplified by each element of the streamis further amplified by a following stream element throughout the activelength of the propagating structure.

It will be apparent that the tube may be made as long asdesired-:toprovide substantially any desired degree of amplificationwithout requiring any corresponding extensionf of the lengths of theelectron paths. The amplified wave appearing at the right hand end ofthe structure 1 is conducted through the lead 39 and the coaxialterminal 43 to suitable utilization means, not shown.

One of the problems which occurs in the design of mostV travellingrwavetubes is that of. providing a broad band impedance: match between theinput and output connectors and` the slow wave propagating structure. Inthe` present device, the bowed conductive members 11 act as extensionsof the outer conductor of the coaxial connector; 41, providing atransition from an ordinary coaxial line to ak multiple conductor openwire line. The' centrali conductor 37 merges into and becomes the iso'-lated single conductor of the helix. This' structure has` been found toprovideV a satisfactory impedance matchthroughout a relatively wide bandof frequencies from about 150: to 350 megacycles per second.

A tube designed substantially as 'shown and of approxima-tely thesizeshown.` n Fig. l` has been found to provide a gain ofabout 30 db atZOO-megacycles, With a` beam voltage ofSOl volts and a beam current of60 milliamperes. No attenuating means is required to preventbackward-amplification and feedback, although the helix itself hasextremely low insertion loss.

If the tubeiis to be designed to provide relatively high power gain thecathode 19 may be arranged to emit more electrons as the output end isapproached. This may be` done by making the cathode strip larger nearthat end, or` by using. a separate larger cathode assembly where moreemission isl required.

Figs. 4, and 6 show the internal details ofv av tube similar to that ofFig. l but embodying a different wave propagating, structure comprisinga conductivebar orl block 51 provided with a series of obliquelydisposed slots S3. The cathode assembly 17 and collector electrode 31may be the same as the correspondingly designated elements of Fig. l. Aninput wave guide 55 is connected to one end of the bar 51, with theinterior of its lower wall 57 flush with the slotted surface. An outputguide 59 is connected similarly to the other end of the bar 51. 7Theslo'ts 53 are made of progressively greater depths from` the ends of thebar 51 toward the central active portion which is traversed by theelectron stream, where they are of uniform, depth. The. upper walls 61and 63 of the wave guides 55 andv 59 extend to the active portion of thewave propagating. structure, and may be made' tov'c'over this part also,forming one continuous wall from` one end of the tube to the other.

The operation of the device of Fig. 4 is essentially the same as that ofFig. 1. The slots 53 act as lumped reactive. loadingl elements, loweringthe. velocity of the wave,` propagation along the bar 51. However, thepropagation velocity lengthwise of any sloty is substantially the sameas. that. of light in free space. As in the structure ofl Fig. 1, aiwave travelling along the member 51 willhave, its. phase front turnedobliquely withy respect to the longitudinal axis ofthe member 51, andthe average electron. velocity ismade substantially equal to thetransversier component of wave velocity. The amplified waveV at theupper end (in Fig. 4) of each slot 5'3 appears at lthe lower end. of thesame slot, and acts on a different part of the electron stream.

The structure of Fig. 4 has somewhat different opera tional.characteristics from that of Fig. l, and may be preferred. for extremelyhigh power and high frequency applications. The nature of the retardingstructure (i. e. theslotted bar) is such that the velocity of wavepropagation-,will dependupon the wavelength or frequency, and the;bandwidth throughout which uniform gain. can he` obtainedwith a givenadjustment of the acceleratingvoltage isy accordingly limited by thischaracteristic, which is calledl dispersion Asan alternative to theabove described mode of operation, theY device of` Fig. 4 may beoperated in a space harmonic mode, wherein the delay or retardationcaused by each slot 53 is much less than the relatively large amountrequired to make the velocity of a given phase front match thatof theelectron stream. Ordinarily, the delay per slot must be suchl that thetime required for a wave. to1 advance. from one slot to the next is thesame as that required. for an electron to travel from one slot to'l thenext.k However, withy a repetitive structure, if in the time an electrontakes to goV from one slot to` the next, the. wave goes the distancebetween slots plus one full' waveelength, the' electron will besynchronizedL with a travelling. field component of much lower' velocitythan the waves, fundamental: mode velocity.

The space harmonic type of operation may be preferred where itisinconvenient. or impractical to construct a retardingstructurei tooperate in the fundamental mode, for example in tubesintended for highpower or extremely highfrequency operation. It will be apparent that thedevicesopjerated in. space harmonic modes will be relativelyfrequency-sensitive, but this may. be advantageous under somecircumstances.- It should be noted alsol that thel skewedy helix typeof. structure shown in Fig. l' may be operated in space harmonic modes,if desired.

The narrow cross section of the propagating structures in the devices ofFigs. 1 and 4, and the fact that the electronstream is directed alongthe sides thereof, makes it practicaltoiuseV a= stacked arrangement likethat shown iniFig. 7. Hereapair of helices 1 and1 are'provi'ded onoppfbside` sides of the space through which an electron streamisdirectedrfrom a cathode 17 to a collector 31.

The helix windings are skewed in the samedirection, and' areoperatdinf'parallelf, i. e. the two input ends are. connected'-tegetner, and so are the two output ends; Various (Jlhr'ta'lglrltmgements"v will be apparent; plural eleoteraction space.

ated-a tron streams may be used, for instance, and other propagatingstructures such as that of Fig. 4 may be combined.

y Fig. 8 shows a double-stream travelling wave tube embodiment of thepresent invention. In this device a skewed helical winding 1" isprovided at the input end of the tube. The Winding 1" is like theinitial part of the winding 1 in Fig. l, but it stops near the activeportion of the tube which is traversed by the electron stream. A secondwinding 1'", similar to the final part of the winding 1, starts near theactive portion and continues to the output end of the tube. A cathodeassembly 17 is provided at` the input end, and a collector 31 isarranged at the output end to cooperate with the cathode 17'.

The cathode 17' provides a thin sheet like electron stream flowinglengthwise of the tube to the collector 31', and passing in closeproximity to the sides of the skewed helical windings 1" and 1. Theinput Winding 1" is excited, as in the tube of Fig. l, by the wave to beamplified. The electrons in the longitudinally flowing stream arebunched by the action of the field of the helix 1", the bunches being inthe form of lines parallel to the turns of the helix. As the obliquelinear bunches travel through the interaction space, they pass throughthe transverse stream from the cathode 17 in such manner that anelectron in said transverse stream interchanges kinetic energy withsuccessive portions along a given lineal bunch in the longitudinalstream.

The transverse stream itself becomes bunched, and give up energy to thelongitudinal stream, intensifying the bunches therein continuously as ittravels across the in- The intensified bunches then pass over the helix1', inducing a wave therein like the input wave, but of largeramplitude. The amplified wave may betaken off and utilized as in thesystem ofFig. l.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description and shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

l. An electron discharge device, including a wave propagating structurecomprising a series of conductive elements disposed in side by siderelationship and defining a common planar surface of substantiallyuniform width and of a length greater than said width; said elementsbeing coupled to effect wave propagation along the length of saidstructure from one end of said series to the other, and means includinga lineal electron gun adjacent a lateral edge of said surface forproducing a sheet-like stream of electrons which substantially conformsto said surface and is closely adjacent thereto for interaction withwave propagation along said conductive elements, said means includingfocussing and accelerating electrodes for directing said electronsacross said wave propagating structure adjacent said surfacetransversely to the direction of wave propagation and at less thanninety degrees to the maj-or dimensions of said conductive elements.

2. An electron discharge device, including electromagnetic Wavepropagating means supported along an axis, said means including aplurality `of spaced conductive element portions disposed insubstantially parallel relationship transverse said axis in a comm-onplane parallel with said axis, and means including a lineal electron gundisposed laterally of said conductive element portions, said last-namedmeans including a cathode assembly for producing and directing asheet-like stream of electrons across said axis in a plane substantiallyparallel with and closely adjacent said common plane for interactionwith wave propagation along said conductive element portions, saidelectrons travelling in a direction transverse to the direction of wavepropagation along said propagating means and at less than ninety degreesto the major dimensions of said conductive element portions.

, 3. An electron discharge device as set forth in claim 2,- furtherincluding at least one further cathode assembly adjacent and incooperative relationship with said Wave propagating means for producinga second stream ofelectrons along the axis of said wave propagatingmeans.

4. The invention as set forth in claim 3, wherein said further cathodeassembly is adjacent the part of said wave propagating means which isfirst traversed by a wave travelling thereon, and said first cathodeassembly is adjacent a part of said propagating means which issubsequently traversed by said Wave.

5. An electron discharge device, including a wave propagating structurecomprising a series of lineal conductive elements disposed in parallelside by side relationship, and defining a common planar surface, meansconnecting said elements to eect wave propagation along said structurefrom `one end of said series to the other,

means including a lineal electron gun adjacent an edge of said surfacefor producing a sheet-like stream `of electrons which substantiallyconforms to said surface, said last named means comprising means fordirecting said stream across said wave propagating structure closelyadjacent said surface for interaction with wave propagation along saidlineal conductive elements, said stream travelling in a directiontransverse to the direction of said Wave propagation in said structureand at an angle of less than ninety degrees to the major dimensions ofsaid conductive elements, means for applying wave energy to be amplifiedto one of said ends of said wave propagation structure, and means forleading amplified Wav energy away from the other of said ends.

6. The invention as set forth in claim 5, wherein said wave propagatingstructure is of oblong cross section, with one of its broad sidesconstituted by said conductive elements.

7. The invention as set forth in claim 6, wherein said Wave propagatingstructure comprises a helically Wound conductive wire, with a portion ofeach turn of said wire constituting one of said elements.

8. The invention as set forth in claim 7, wherein the turns of said wirein said helical winding are oriented obliquely with respect to theprincipal axes of said wave propagating structure.

9. An electron discharge device, including a wave propagating structurecomprising an elongated bar of conductive material having a plurality oftransverse slots providing a connected series of lineal conductiveelements disposed in parallel side by side relationship to effect wavepropagation along said structure from one end of said series to theother, and defining a common planar surface, means including a linealelectron gun adjacent an edge of said surface for producing a sheet-likestream of electrons which substantially conforms to said surface, saidlast named means comprising means for directing said stream across saidwave propagating structure closely adjacent said surface in a directiontransverse to the major dimension of said structure and at an angle ofVless than ninety degrees to the direction of said slots for interactionwith wave propagation along said lineal conductive elements, means forapplying wave energy to be amplified to one end of said wave propagationstructure, and means for leading amplified wave energy away from theother end of said structure.

l0. The invention as claimed in claim 9, wherein said slots are obliquewith respect to said major dimension of said wave propagating structure.

ll. The invention as claimed in claim l0, wherein said slots are ofsubstantially uniform depths throughout the portion of said wavepropagating structure which is crossed by said electron stream, and areof gradually decreasing depths from the ends of said portion to the endsof said structure.

l2. An electron discharge device including means for propagatingelectromagnetic energy along an axis in the form of Waves with theirphase fronts inclined with re- 7 spect. to 'saldi axis, said meanshaving anl oblong cross section, and means for producing and' directing'a' stream of elec rons or oblong 'cross section transversely acrossvsaid for encountering said waves and interact-ion therewith atprogressively diierent' space phases from one S'i'de of said propagatingmeansto the other, said elect'rons having a vvelocity componentperpendicular to said phase fronts, the narrow dimension of the crosssection of said electron stream being orientedl substantially parallelwith 'the narrow dimension of the cross section of said electromagneticenergy propagating means and the wide dimension. -o'f the cross section'of said electron stream being 'oriented at' an angle withr respect tothe wide dimensien of the croSssect-ion of said wave propagating means.

The; invention as set forth in claim 1'2", further including meansv forproducing and'l directing a second stream ofj electrons along said axis,said second stream being ofI substantial extent transversely of itsdirection of motion, saidpropagating means comprising rst and secondsections with saidv rst section comprising means for modulating said`second stream to form` transverse linear bunches therein, said bunchesbeing obliquely inclined withr'espect t-o said axis and with respect tothe direction of motion of the electrons in said lirs't mentionedstream.

14. An electron discharge device, comprising an evacuajted@ tubularenvelope, a conductive helix of oblong cross section supported in saidenvelope, the longitudinal axis of! said helix extending along the axisof said tubular envelope, the respective ends -of said helix comprisingin put and output terminals for coupling to input and outputelectromagnetic transmission line structures, respectively, anelect-'ronl gun supportedy within said envelope adjacent one of thenarrow sides of said helix for producing and directing an electron beamof oblong cross section transverse the axis of said helix and across oneof the wide sides of said helix with the beam electrons travelling at anangle with respect to the adjacent helicalV turn portions of said'- onewide side in energy exchanging relationship with said turn portions fromone to the yother edges of said one' wide side ofsaid helix, the narrowdimension of the cross section of said electron beam beingorientedsubstantially parallel with the narrow dimensionl of the crosssection of said helix, and collector electrode means supported withinsaid envelope adjacent the other of the narrow sides of said helix forreceiving said electron beam from said electron gun.

115. An electron discharge device as set forth in claim 174, wherein theturns yof said helix adjacent said electron beam are` skewed withrespect to the axis of said helix.

16. Anelectron discharge device, comprising a slow waveelectroniagneticv wave energy propagating structure having a longitudinaily axis", saidv structure ccrnpri'singl means ferk guiding microwaveenergy from one region to another` region spaced? from said: oneV regionalong said longitudinal axis, said propagating Astructure including 'aplurality of conductive' elements disposed" transverse said axis witlrportions of saidi elementsv lying substantially in a commonplaneparal-lel with said axis, and means in-l cluding a strip*y cathodemember disposed= laterally of said propagating structure for producingand directingl a stream of electrons of substantially rectangular crosssection in a transverse direction relative to saidcon'ductive elementportions and the off said propagating structure for modulation byelectromagnetic wave 'energy along said conductivegelement's.

17. An electron discharge4 device, comprising a travel'- lingelectromagnetic wave helix of oblongcross section, said helix having alongitudinal axis andr said helix inw cluding a plurality of adjacentlineal turn elements lying ina common plane, andtmeans including alineal electron gun disposed laterally of said helix for producing anddirecting an electron stream of substantially rectangular cross sectioninE a transverse direction relative to said longitudinalalxis of saidhelix andy the lengths of said lineal turn elements.

183. electron discharge device as set forth in' claim 17 wherein saidlineal turn elements arel substantially' parallel with each other and inan oblique relationship relative to-said axis.

19; A11l electron discharge device as set forth in claim 18', whereinsaid'c lineal turn elements of said helixv are inI a plane-parallelwiththe direction ofVv electrons from said' electron gun, and saidlineal' electron gun includes a stripf cathode having a substantiallyplanar faceof rectangular` crossys'ecticmvin a plane at right' angles to'said plane of said lineall turnV elements.

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